It doesn't matter if chemical warfare was a war crime at that time...because the chemical wasn't dropped to cause cancer and birth defects, as has happened, it was dropped as a defoliate. In essence, it was dropped for it's intended use, the same use as farmers use Round Up, to kill plants. The only difference is that it was dropped by soldiers from planes, on civilians.
But, it does matter who ordered what. Did Monsanto know of the possibility of birth defects and cancers? Did the government? Who ordered it? There certainly needs to be an extensive investigation.
It might not have been a war crime at the time...but it is still a horrendous act.